Jet pump diffusers in a nuclear reactor, such as in a boiling water nuclear reactor, typically form part of a jet pump and are utilized to maintain the floodability of the reactor core at a safe level. The jet pump diffuser also typically discharges water from an inlet mixer into the lower core plenum to maintain adequate recirculation flow.
One known jet pump diffuser has a generally conical shape, and the larger diameter end of the diffuser is welded to an adapter assembly engaged to an Inconel nickel-chromtun alloy shroud support plate. The smaller diameter end of the diffuser forms a slip fit connection to the inlet mixer section of the jet pump.
Water, generally under high pressure, flows through the jet pump diffuser from the smaller diameter end to the larger diameter end. The water exits the diffuser at the larger diameter end and is discharged through the adapter assembly into the lower core plenum. The discharged water mixes with the water in the lower core plenum and may cycle through the reactor.
As the water flows through the jet pump diffuser, the speed and pressure of the water decrease due to the increasing diameter of the diffuser. Decreasing the water speed and pressure just prior to being discharged into the lower core plenum improves the mixing and flow characteristics of the discharged water.
Of course, the integrity of the welds in the jet diffuser and adapter assembly is important. A crack or corrosion in such a weld, for example, if unrepaired, could result in reduced jet pump flow. Therefore, jet pump welds may be inspected during reactor shutdown to ensure the integrity of such welds.
In the event that crack indications are ever found in such a weld, the repair assembly preferably will locally retain the region of the crack to provide the support necessary to maintain the diffuser and adapter assembly secure during all possible operating conditions, including accident events. For example, during a seismic event, vertical and horizontal motions may be generated and transmitted throughout the reactor vessel structure including the jet pump assemblies. The repair assembly preferably should be able to withstand such vertical and horizontal motions and retain the diffuser and the adapter assembly in an operative position. Further, the repair assembly preferably would provide such necessary retention and support even for a complete circumferential crack in the weld joint. In addition, the repair assembly preferably could be installed without having to remove any components of the jet pump, e.g., the inlet mixer. If components of the jet pump have to be removed, such component removal would increase the repair costs and time required to perform the repair.